Our studies concern the lymphotropic human herpesviruses 6 and 7 (HHV-6 and HHV-7). HHV-6 causes roseola infantum, a childhood disease characterized by high fever and skin rash. Human herpesvirus 7 (HHV-7) was isolated in our laboratory from CD4+ T cells of a healthy individual. Roseola infantum is generally a self limiting illness and virus can only be recovered during the acute phase of the disease. The virus then undergoes into a latent phase inasmuch as peripheral blood of healthy individual harbor viral DNA. Recent evidence has suggested that viral reactivation may occur in patients undergoing immunosuppression, such as in the course of bone marrow, liver, and kidney transplantation. Furthermore, the virus can cause serious disease, inasmuch as recent reports have provided evidence for viral involvement in central nervous system in roseola patients with encephalitic complications. It is thus of interest to study parameter which determine the interaction of the virus with the host cell, whether fully productive, persistent or latent. Our studies of this past year concerned genome structure and the layout of viral genes, as a prelude to analyses of gene function and regulation. These studies included the cloning of 60kb regions of HHV-7 DNA, sequencing close to 5 kb, mapping of HHV-6 DNA and analyses of heterogeneous viral DNA regions in a number of virus strains. A second avenue of research included studies related to virus-host cell interactions, with the goal of delineating parameters which affect and influence the course of infections in the human host. Studies in this category included analyses of the effect of T cell activation on virus replication, activation of HHV-6 and HHV-7 from latency, analyses of the effect of virus infection on T cell function, and analyses of parameters which determine the outcome of infection whether latent, recurrent, persistent or fully productive infection.